1. Field of the Invention
The present invention relates to a communication apparatus for a vehicle that is adapted to be applied to a vehicular electronic key system, etc., and more particularly, it relates to a suitable technique when an electronic key is applied to a passive entry system in which the electronic key operates as a portable unit to lock and unlock doors of a vehicle in a simple and easy manner.
2. Description of the Related Art
As a conventional communication apparatus for a vehicle, there has been known a vehicular lock control apparatus which uses an electronic key as a portable unit (see, for instance, a first patent document: Japanese examined patent application publication No. H03-60696).
The vehicular communication apparatus or vehicular lock control apparatus as described in the first patent document is provided with a portable unit (transmitter-receiver) having a microcomputer and a memory built therein, and the portable unit further has a transmitting and receiving antenna, a reception and demodulation circuit, a modulation circuit, and a carrier generation circuit.
In addition, a receiving section, an intrinsic signal verification section, a steering lock actuator, a lock state detection section, a portable unit detection section, a engine stop detection section, an unlock control unit, and a lock control section are arranged as telecommunications equipment on a vehicle side.
Now, reference will be made to the operation of the conventional vehicular communication apparatus as described in the first patent document.
First of all, on the vehicle side, the receiving section receives a predetermined intrinsic signal radio transmitted from a portable unit that has received a request signal from the vehicle side.
The intrinsic signal verification section verifies the intrinsic signal thus received with an intrinsic signal registration value that has been registered beforehand. When the intrinsic signal coincides with the intrinsic signal registration value, the lock state detection section detects a steering lock state at a time point when the verification is performed.
At this time, if a steering lock state is detected (i.e., the steering of the vehicle is in a locked state), the unlock control unit controls the steering lock actuator so as to carry out a steering unlocking operation.
In addition, when it is detected by the portable unit detection section that the portable unit is outside the passenger compartment of the vehicle, and when an engine stop state is detected by the engine stop detection section, the lock control section controls the steering lock actuator so as to carry out a steering locking operation.
On the other hand, in the portable unit, the request signal from the vehicle side is first received by a transmitting and receiving antenna, and is then demodulated by a reception and demodulation circuit to be input to the microcomputer, so that it is determined whether the signal thus input is a request signal.
When the input signal is a request signal from the vehicle side, a code signal inherent to the vehicle (portable unit), which has been stored in the memory beforehand, is read into the portable unit, and a pulse train signal corresponding to the intrinsic code signal thus read is impressed to the modulation circuit.
As a result, the intrinsic code signal modulated by a carrier wave output from a carrier oscillation circuit is sent from the transmitting and receiving antenna to the vehicle side.
In the conventional communication apparatus for a vehicle, there is a problem that the oscillation circuit need be built in the portable unit, and a power supply and a power supply circuit for driving a transmission and reception circuit (the oscillation circuit, the modulation circuit and the demodulation circuit), the microcomputer and the memory are also required.
In particular, though a battery can be used as the power supply for the portable unit, power consumption is large in the vehicular communication apparatus in which frequent communications are carried out between the portable unit and the vehicle side, so there is a problem that the battery life is short and the frequency of battery replacement becomes high.
Further, there is also a problem that the area occupied by the power supply circuit becomes large, thus resulting in an increase in size of the portable unit.